The present invention relates to a method for automatically focusing an optical system, especially a microscope, making use of a scanning grating, and to the apparatus for implementing this method.
It is known to automatically focus optical apparati by mounting a scanning grating (sensor) in the image plane of the system and acting as a spatial frequency filter, the spatial freqeuncy corresponding to the grating constant being filtered out of the intermediate image and being used through the intermediary of a photodetector to generate electrical signals. The grating in this method is moved over the intermediate image, and different signals depending on the focus or lack of it in the system are obtained which represent a criterion for the required focus adjustment.
The state of the art of focusing detectors and the gratings useful therein may be ascertained by reference to U.S. Pat. No. 3,781,110, the disclosure of which is incorporated herein.
New sites are found, especially in microscopes, for which the sensor can generate only a small signal, or virtually none, when the grating is made to oscillate only across a few grating constants or when the microscope object is displaced, so that focusing is impossible at those sites, or even defocusing takes place. In a manner escaping the naked eye, these sites of the preparations are characterized in that the light fluxes measured by the two photodetectors are equal when a prismatic grating is used as the scanning grating, or in other words, the spatial frequency corresponding to the scanning grating cannot be detected in the object for the particular present relative motion between object and grating. The second formulation also applies to amplitude gratings operating as scanning gratings. Good signals may occur for a fairly small displacement of the intermediate image.
Even though this phenomenon is found in the automatic focusing of microscopes, it must necessarily also occur in the focusing of other optical apparati, for instance in self-focusing photographic cameras.
The state of the art of automatic microscope focusing devices may be ascertained by reference to U.S. Pat. Nos. 3,731,827 and 3,798,449 the disclosures of which are incorporated herein.